docs: def import_env(*envs):
      'import environment variables from host'
      for env in envs:
          parts = env.split(':', 1)
          if len(parts) == 1:
              export_as = env
          else:
              env, export_as = parts
          env_val = os.environ.get(env)
          if env_val is not None:
              yield '{}={}'.format(export_as, shlex.quote(env_val))